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Evolution of Staph Superbug Traced Between Humans and Livestock

A strain of the potentially deadly antibiotic-resistant bacterium known as MRSA has jumped from livestock to humans, according to a new study involving two Northern Arizona University researchers.

Paul Keim, Regents’ professor and director of NAU’s Center for Microbial Genetics and Genomics, and Lance Price, NAU faculty member and director of the Center for Food Microbiology and Environmental Health at the Translational Genomics Research Institute, collaborated with scientists at 20 institutions around the world on the study published today in the online journal mBio.

The TGen-led research utilized whole genome sequencing to study 89 genomes from humans and animals—including turkeys, chickens and pigs—with samples from 19 countries on four continents.

The research focused on methicillin-resistant Staphylococcus aureus CC398, also known as pig MRSA or livestock-associated MRSA because it most often infects people with direct exposure to swine or other livestock. It is likely that MRSA CC398 started as an antibiotic-susceptible strain in humans before it jumped to livestock.

After transferring to livestock, MRSA CC398 became resistant to two important antibiotics, tetracycline and methicillin, which are used for treating staph infections. The resistance likely is a result of the routine antibiotic use that characterizes modern food-animal production. The animals commonly are given antibiotics to prevent infection and promote growth.

Keim, who also serves as director of TGen’s Pathogenic Genomics Division, said the study describes evolution in action. "The most powerful force in evolution is selection. And in this case, humans have supplied a strong force through the excessive use of antibiotic drugs in farm animal production. It is that inappropriate use of antibiotics that is now coming back to haunt us.”

Price, the study’s lead author, said the research was “like watching the birth of a superbug—it is simultaneously fascinating and disconcerting.” He said that while this strain of MRSA was discovered less than a decade ago it appears to be spreading very quickly.

“Our findings underscore the potential public health risks of widespread antibiotic use in food animal production,” Price said. “Staph thrives in crowded and unsanitary conditions. Add antibiotics to that environment and you’re going to create a public health problem.”